On aluminium surfaces there is always oxide present. The native film is 3 to 10 nm. On pure aluminium the oxide is amorphous alumina (Al2O3), while for example on a Mg-containing alloy the film contains a large amount of MgO.
The oxide film influences many properties of the metal, e.g. the corrosion.
In terms of the optical properties, the oxide’s thickness and composition are important and can be changed via surface treatments. In these experimental examples, the oxide thickness has been increased via anodising . The presence of minima or maxima in the TR are discussed later. Generally, the higher the total reflectance in the visible light range (400-700 nm), the brighter the surface appears.
Use the slider bar to change the oxide thickness and observe the effect on the total reflectance (TR).